(a) Field of the Invention
The present invention relates to a display and a driving method thereof. More specifically, the present invention relates to an organic electroluminescent (EL) display and a driving method thereof.
(b) Description of the Related Art
In general, an organic EL display is a display device for electrically exciting fluorescent and organic compounds and emitting light therefrom, and voltage or current drives (M×N) organic emission cells to represent images. An organic emission cell includes an anode (indium tin oxide: ITO), an organic thin film, and a metallic cathode layer. The organic thin film includes an emission layer (EML), an electron transport layer (ETL), a hole transport layer (HTL) for balancing electrons and holes to improve emission efficacy, and additionally includes an electron injection layer (EIL) and a hole injection layer (HIL).
Methods for driving the above-configured organic emission cells include the passive matrix method for allowing anodes and cathodes to cross each other according to an addressing method, selecting a line, and driving the line, and the active matrix method for connecting a thin film transistor (TFT) and a capacitor with each ITO pixel electrode so as to maintain a voltage by a capacitance. The active matrix method includes a voltage programming method and a current programming method according to signal types (a voltage or a current) used by a driving circuit.
An organic EL display comprises an organic EL display panel, a scan driver, and a data driver. The organic EL display panel includes a plurality of data lines for transmitting data signals that represent image signals, a plurality of scan lines for transmitting select signals, and pixel circuits each formed at a pixel area defined by two adjacent scan lines and two adjacent data lines. When the scan driver applies a select signal to a scan line, a transistor is turned on by the select signal, data signals for representing image signals are applied through the data lines to a gate of the transistor from the data driver, and a current flows to an organic EL element through the transistor to emit light.
In order to reduce the number of data driver ICs, demultiplexers or shift registers are used, and a current sample/hold circuit is used when the panel is driven according to the current programming method. It is usual for the current programming method that requires much time of programming data to pixels to use the current sample/hold circuit so as to realize high resolution. However, since the conventional sample/hold circuit uses TFTs as active devices for sampling and holding the current, the available current range is limited because of the TFT characteristics. Also, the sampled current and the held current are different because of the kickback phenomenon that is caused when switching the TFTs, and the output current of the driver IC and the current for driving the data lines become different.
Control the output current of the driver IC is then so as to eliminate the current variation, and hence, a gamma correction process is needed. A deviation of the hold current caused by the deviation of the TFT characteristics is generated during the gamma correction process, and the images may not be uniform.